Process for preparing arsenates by anodic oxidation of salts of arsenious acid



May LOWENSTEIN 2,375,933

PROCESS FOR PREPARING ARSENATES BY ANODIC OXIDATION OF SALTS OF ARSENIOUS ACID Filed DEC. 12, 1938 \6 NICKEL STRIP 5 N/OKEL STRIP 9/ 1 NICKEL SHbtT I; 5 7 I :f; NICKEL W000 BLOCK f SHEET 1-? 4* N/CKELED/RON SHEET -70 .1; 5

" N/C/ffL SHEET If; N/GKEL SHEET 3- 5;; 1 E3}.'-- 2 NICKEL GAUZE NICKEL GAUZE ASBESTOS .1; PAPER Patente May 15, 1945 PROCESS FOR PREPARING ARSENATES BY ANODIC OXIDATION OF SALTS F AR- SENIOUS ACm Leo Liiwenstein, Berlin-Wilmersdorf, Germany;, vested in the Alien Property Custodian ApplicationDecember 12, 1938, Serial No. 245,310 In Germany Eecember 13, 1937 2 Claims.

It has been found in the electrolytic preparation of arsenate solutions that the yield figures vary according to the nature of the arsenite employed. Thus for example in the case of the electrolytic production of sodium arsenate or potassium arsenate the yields are as low as up to 40% and less, according to the nature of the starting product. Even when heating, the tensions rise strongly up to round about one third of the total tension.

It has now been found that it is possible to operate with raw substances of any origin with obtention of almost theoretical current yields and employing relatively low tensions if operations are carried out in the following manner:

To the anolyte are added soluble compounds of metals which are capable of several oxidation stages. For example additions of chromium, iron, lead, manganese and the like may be used. These metals were present in a quantity of the order of 1 g. per 100 cos. of solution, and even in smaller concentration. However, only those metals may be used which are not precipitated out by the anolytes coming into question.

It has also been found that these metals possess varying degrees of efiiciency. It was found that for example chromium is particularly effective, whilst iron is less so, and lead still less.

Erample.'-In an electrolyser with diaphragms there is a caustic soda solution in the cathode space whilst the anode space contains a 19% sodium arsenite solution. Electrolysis was carried out at a temperature of 60 C. No solid salt separates out. The sodium arsenite had been produced by dissolution of commercial arsenic in caustic soda solution. The current yield amounts to approximately 50%. Similar results are obtained if potassium arsenate is prepared in the same way from the same arsenic.

If now the electrolysis is carried out in the same way with addition of dichromate to the anolyte, so that there are 0.2 g. of dichromate in 100 ccs., then the yield amounts to more than 98%,

Whereas without this addition the tensions vary approximately from 2.7-3.1 volts, after the above addition of dichromate they amount at the same temperature to approximately 2.2-2.5 volts.

No dichromate was contained in the arsenate crystallising out on cooling down. The dichromate remained completely in the mother liquor, which was returned to the electrolysis.

It has been found that the apparatus shown in the accompanyin drawing is particularly suitable for carrying out the electrolysis.

against some layers of asbestos paper'2 separating them. Electrolysis is now carried out in accordance with the example given above. The nickel sheets la and 3a serve for supplying the current. The anode lead consisting of nickel sheet is indicated by 5 and the cathode lead, likewise consisting of nickel sheet, is indicated by 6. '2 represents a wood block, e. g. discshaped. 8 represents a nickel sheet running round the interior in the form of a ring, whilst 9 indicates a nickel sheet running round the exterior in the form of a ring.

It has been found that after fairly prolonged use some arsenic salt solution also reaches the cathode and is there reduced to metallic arsenic. This floats in the cathode space in the form of dark particles and can be readily removed from this space by circulation of the catholyte. It may be avoided if a continuous small flow of catholyte into the anode space is provided for by having a somewhat higher level for the catholyte, amounting approximately to about 1 cm. This also has the efiect that on crystallising out the arsenate this comes out further than in the less alkaline solution.

A cylindrical form for the cell has proved to be very advantageous. On a wire cylinder first of all several layers of asbestos paper are wound and then a further wire cylinder is slipped thereover. One end 01 the two cylinders is closed, e. g. by means of a round disc of gauze or wire netting, between which end pieces there is a disc of asbestos. A suitable number of such cells (l0, 16, 20 or more connected in 'parallel) are placed closely together in a box 4 (electrolyser) of nickled iron sheet.

In a cell'of this kind for 100 amperes with which the inventor has worked, the inner gauze cylinder was cm. high and the outer gauze cylinder 76.5 cm. high. The internal diameters of the inner and outer cylinders amounted, to 20 and 20.5 cm. respectively. Each cylinder had a gauze bottom. The current lead Ia was 5 cm. wide, 48 cm. long and 0.5 mm. thick; the current lead 3a had the same dimensions. The leads 5 and 6 were each 5 cm. wide and 1 mm. thick, the

nickel sheets 8 and 9 each 4 cm. wide and 1 mm. thick. In between the two cylinders there were several layers of asbestos paper. The current yield of practical continuous Working with such a cell was not below 98%, the voltage 2.2. The dichromate in the anolyte was maintained at 1.6%. from the electrolyser did not in the crystallisinu, vessel go into the crystallising arsenate but remained wholly in the mother liquor.

Whilst in the foregoing dimensions have been given of a cell which the inventor has himself used, it is to be distinctly understood that the invention is not limited to cells of these particular dimensions.

What I claim is:

1. In the process for preparing an alkali metal The dichromate in the anolyte coming arsenate selected from the group consisting of sodium and potassium arsenate which comprises electrolyzing as the anolyte in the anode comartment of an electrolytic diaphragm cell an aqueous solution selected fromthe group consisting of sodium and potassium arsenite respectively, while providing a solution in the cathode compartment selected from the group consisting of caustic soda solution and caustic potash solution respectively, the improvement which comprises carrying out the electrolysis in the presence of a soluble dichromate that is not precipitated by the anolyte.

2. The process of claim I in which the dichromate is present in an amount about 1.6% of the anolyte.

LEO LOWENS'I'EIN. 

